An ability to form planar lightwave circuits (PLCs) of great complexity within a reasonably small area has led to the development of waveguide-based devices and systems across many applications, such as telecommunications, data communications, radiation sensing, and chemical detection. In telecommunications and data communications systems, for example, PLCs including signal couplers, splitters, wavelength-based routers, and the like, have become seemingly ubiquitous. In chemical detection applications, chemical sensors are widely used to detect the presence and/or concentration of one or more chemicals in applications such as homeland defense, biological and chemical warfare detection systems, and pollution monitoring.
For many devices, more than one wavelength of light is conveyed through the PLC. In wavelength-division multiplexed telecom systems, for example, many different wavelength signals are used, each carrying voice or data information. In chemical sensing applications, for example, different wavelength signals can be used to signify the presence of different chemicals.
In many cases, coupling light between a surface waveguide and an external component, such as an optical fiber or bulk optic element, can be problematic. In telecommunications (and data communications) systems, this problem is mitigated by employing surface waveguides that are single-mode. Single-mode waveguides typically have a very small waveguide core that has a width and height that are substantially the same. As a result, light enters and exits the waveguide with a narrow Gaussian-shape that couples very well with external components.
For chemical sensors, on the other hand, it is often desirable to use a waveguide that has highly asymmetric core layer, wherein the light guiding region is very thin in the vertical dimension (e.g., <1 micron) but very wide horizontally (e.g., >100 microns). Such a waveguide is often referred to as a slab waveguide. Typically, light propagating through the core has an evanescent field that propagates in the cladding layers below and above the core layer. A chemically sensitive material is disposed on the upper cladding layer. When in the presence of a target chemical, the chemically sensitive material alters the evanescent field, which changes a property (e.g., amplitude, phase, etc.) of the light propagating through the core. This change in property constitutes an output signal that is based on the presence of the target chemical. A thin slab waveguide facilitates interaction between the chemically sensitive material and the evanescent field. A wide slab waveguide enables reasonably large sensing regions as well as increasing the amount of light that propagates through the sensing region.
Unfortunately, the light emission of a slab waveguide is non-Gaussian. As a result, beams that enter and exit slab waveguides are poorly matched to external optical elements. Further, slab waveguides are typically multi-mode, which exacerbates these issues.
Many chemical sensors employ a fluorescent material disposed on the top cladding layer. The fluorescent material is “armed” (i.e., stimulated) by an excitation signal comprising light at a first wavelength (i.e., an excitation wavelength) by propagating the excitation signal through the waveguide core. Light in the evanescent field of the excitation signal is absorbed by the fluorescent material, which puts it into an excited state. When exposed to a target chemical, the excited fluorescent material generates an output fluorescence signal at a second wavelength (i.e., a fluorescence wavelength). The target chemical may be an individual chemical, a chemical compound, an analyte, or a biological substance, for example.
Unfortunately, reliable detection of the fluorescence signal can be difficult. Often, the fluorescence signal results in only a slight change in overall intensity of light received at a photodetector. It can be problematic, therefore, to differentiate between noise due to fluctuations of the light source used to provide the stimulative light from the fluorescence signal itself. This low signal-to-noise ratio limits the overall sensitivity of many prior-art fluorescence-based sensors.
In order to improve detection of the fluorescence signal, spectral filters have been used to block the excitation signal at the photodetector. Unfortunately, there is typically only a slight difference between the wavelengths of the stimulative light and the fluorescence signal. As a result, the formation of a filter that passes the fluorescence wavelength but not the excitation wavelength is extremely difficult and typically quite expensive.
In many cases, arrays of fluorescence-based chemical sensor regions formed on a plurality of slab waveguides are used, for example, to enable detection of a plurality of chemicals. Excitation and fluorescence signals are typically coupled into and out of the slab waveguides using lenses or diffraction grating elements. A detector, such as a CCD array, is then used to detect the fluorescence signals from the sensor array. Unfortunately, cross-talk between the regions can make it difficult to differentiate one fluorescence signal from another.